The devastation that stroke inflicts is easily found in the medical literature: stroke strikes someone in the United States every 45 seconds and kills someone every three minutes. Each year about 700,000 people suffer a stroke. About 500,000 of these are first attacks. Of all strokes, 88 percent are ischemic, nine percent are intracerebral hemorrhage, and three percent are subarachnoid hemorrhage. The Framingham Heart Study showed that, six months after their strokes, 50 percent of ischemic stroke survivors studied over age 65 had some hemiparesis, 30 percent needed help walking, 26 percent were dependent for daily living, and 19 percent had aphasia. Twenty-six percent were institutionalized. Eight to twelve percent of ischemic strokes are fatal within 30 days. As the mean age of the population increases, the incidence of stroke is projected to increase. Stroke clearly compromises quality of life for its victims and for society at large. In economic terms, the estimated total cost of stroke in the United States in 2004 is $53.6 billion. Direct costs include $26.5 billion for hospital and nursing home stays, $2.7 billion for physicians and other professionals, $1.1 billion for drugs and other medical durables, and $2.7 billion for home health care. Indirect costs are estimated at $6.1 billion in lost productivity due to morbidity and $14.5 billion in lost productivity due to mortality. For Americans age 40 and older, 1995 data showed the average in-hospital and physician costs were $11,010 for a stroke and $4,940 for trans-ischemic attack (“TIA”).
Although transcranial Doppler (“TCD”) ultrasound has been available for many years as a standard diagnostic modality, and has been shown to have utility in the hands of a skilled user for assessing and monitoring the basal cerebral arteries in the early stroke patient, it has not been widely used in this capacity. This is in part because the demand for this sort of assessment is a recent phenomenon accompanying the advent of thrombolytic therapy, and in part because TCD is difficult at best for the emergency medicine physician or nurse to perform and interpret. In elderly populations, where the technology is greatly needed, frustrating time is consumed in finding signals in many patients. Locating the signal is difficult because of the need to laterally explore each depth one step at a time with single-gate TCD equipment, until flow signals are acquired. Searching for flow can be a tedious and perplexing task. Once a flow signal is acquired, it is confirmed by considering the associated depth, the approximate aim of the probe, and tracing the signal to adjacent vessels as depth is varied to verify the user is indeed on the appropriate vessel. Successful utilization of single gate analog transcranial Doppler is not generally available outside vascular laboratory personnel, and even these ultrasound experts have very mixed reactions to TCD because of the above mentioned difficulties. In summary, these factors combine so that there is a lack of utilization of TCD capabilities in triage and monitoring, which is witnessed by the relatively minor presence of TCD in emergency departments across the United States.
The benefits of rapid and easy to perform assessment of cerebral hemodynamics in the early presentation of a patient with suspected stroke are tremendous, both for the admitting physician and the patient facing a potentially debilitating or fatal stroke. For the physician, there is the early recognition of pathology from hemodynamic observations, such as ischemic blockage, its location, and the accompanying option of thrombolytic therapy. Studies have shown that an untreated occlusion of the middle cerebral artery presents a very poor prognosis for the patient. Hence, knowing about it as soon as possible maximizes the potential for positive intervention. For example, after initiating thrombolytic therapy, monitoring can determine the point in time at which thrombolytic therapy re-establishes perfusion, presenting the possibility of termination of successful thrombolysis in order to minimize risk of bleeding associated with thrombolytic drugs. For the patient, “time is brain.” The success of aggressive therapy such as thrombolysis is partly dependent on its application in the first three hours after stroke onset. These benefits will be appreciated in the emergency department and by the patient to the degree that the assessment of cerebral hemodynamics is rapid and easy to perform.
Recently, a digital Doppler platform has been developed by Spencer Technologies in Seattle, Wash. in which up to 33 sample gates can be simultaneously processed into a “color” m-mode image. The color in the m-mode image is a function of Doppler signature power and detected velocity, in that increases in backscattered power cause the colors, red or blue, to become more intense. The digital Doppler platform is referred to as Spencer Technologies' Power M-mode Doppler (“PMD”). Showing power in this fashion conveys to the user when the Doppler beam is well aimed—that is, intensity of color increases with volume of moving blood in the Doppler sample volume and this indicates when the beam is centered on the blood flow. Thus, the color m-mode display of an ultrasound system having PMD capability provides medical professionals who do not have expertise in ultrasound with a mechanism for easy location (by the operator) of the middle cerebral circulation. A more detailed description of PMD ultrasound systems can be found in U.S. Pat. No. 6,196,972 to Moehring, issued Mar. 6, 2001 and assigned to Spencer Technologies.
Displaying color as a function of signal power at multiple depths offers advantages for an examiner to locate a temporal bone window, or an “acoustic window,” without limiting interrogation to a single selected depth. When employed in assessment of patients at various vascular laboratories, the sonographers reported that PMD TCD was easier to use since it was no longer necessary to seek a window by changing depth. Also they found it unnecessary to listen for a Doppler sound. In fact, an ultrasound system having PMD capability enabled them to first find the optimal temporal window on the PMD display and then to adjust the gate depth for audible spectral display using the PMD depth scale. The PMD system allowed the examiner to find the temporal window without audible Doppler sounds and therefore avoid crashing sounds of probe application to the head. During intraoperative monitoring, the sonographers maintained their position on the window using the color signals present in the PMD image as a guide. When the flow signals were accidentally lost during surgical monitoring, these could be recovered by repositioning the transducer using the PMD display as the only feedback. Therefore, the significant problem of single gate TCD, that is, frustration in locating difficult windows, was reduced.
Although PMD provides an easy method for aiming the Doppler probe, the task of locating acoustic windows and underlying blood flow is still left to the operator when using the PMD system. Thus, although the development of the PMD platform mitigated the problems related to needing highly skilled operators to operate single-gate TCD equipment, the problems are nevertheless still present to some degree.